During forestry operations, tool carriers are used to which a working head for harvesting/felling/stacking has been attached. The tool carrier may be a wheeled or tracked chassis with an arm that is attached in a jointed manner at a slewing ring. The arm is divided into a boom and a stick, which are joined to each other at a joint and which in this way achieve a large working area, through it being possible to fold the arm together and shorten it, or to unfold the arm and extend it. The working head is attached to a tool fixture at the extreme end of the stick in a manner that allows it to be removed, or directly attached at the extreme end of the stick. The working head comprises a frame with means to grip a tree and hold it fixed against the frame. Furthermore, the working head comprises a cutting arrangement to cut the tree. The cutting arrangement may be of the type with a guide bar and chain, a rotating disk, a flywheel, or a cutter. Furthermore, the chassis is provided with a driver's cabin in which a driver sits and operates the forestry machine.
If the ground is sloping, the working head must be directed in towards the trunk of the tree, since the trees grow essentially vertically. The driver maneuvers the arm and places the working head against a tree, after which the tree is cut down. Using prior art technology, the working head can be adjusted, leveled, in a vertical direction seen in the direction of the arm. If the ground is, however, sloping, the working head must be manually adjusted by the driver from the driver's cabin. This alignment against the trunk places high demands on the driver, since the alignment is a time-consuming procedure. In order to achieve as good a cutting as possible, the working head must be essentially vertical even if the ground, and thus also the forestry machine, is sloping.
The term “lateral direction” is here used to denote the direction that is transverse to the direction in which the arm is jointed. This can be described in a coordinate system as the z-direction, if the direction of jointing of the arm relative to the stick is described as the x-direction (horizontal) and the jointing of the arm relative to the chassis is described as the y-direction (vertical).